In recent years, increases in resolution of an imaging device used in an imaging apparatus such as a digital still camera or a digital video camera lead to increases in amount of image data processed by an integrated circuit in the imaging apparatus. To handle a large amount of image data, a higher operating frequency, a larger memory capacity, and the like can be proposed as measures for securing a data transfer bus width in the integrated circuit. These measures, however, directly result in cost increases.
Typically, when all image processing is completed by the integrated circuit in the imaging apparatus such as a digital still camera or a digital video camera, the processed image is recorded in an external recording device such as an SD card. Upon recording, the image is compressed so that image data of a larger image size or a larger number of pictures is stored in the external recording device of the same capacity than in the case where the image is not compressed. A coding scheme such as JPEG or MPEG is employed for this compression process.
In Patent Literature (PTL) 1, image data compression is also extended (applied) to a pixel signal (raw data) received from an imaging device. PTL 1 intends to reduce a bus bandwidth necessary for memory read and write and thus achieve a high-speed operation even in the case where the imaging device is increased in resolution and so a higher load of signal processing is required. Moreover, a fixed length coding scheme is employed in order to secure the bus bandwidth and reduce the amount of compression. This is implemented by a method of calculating maximum and minimum values from pixel data in an arbitrary image area and determining a local dynamic range in the image area. A value obtained by subtracting the calculated minimum value from each pixel in the image area is then quantized with a quantization width corresponding to the determined dynamic range. Fixed length coding of image data is performed in this way.
PTL 2 intends to reduce memory usage and increase the number of continuous shots by compressing raw data, because the number of continuous shots typically depends on the number of pictures of raw data that can be stored in a buffer memory. In PTL 2, too, fixed length coding is employed in order to ensure a continuous shooting rate. This is implemented by a method of calculating a predictive difference between a pixel value (target pixel) to be compressed and a predictive value predicted from known data. When the calculated predictive difference exceeds a predetermined threshold, a quantization step (quantization width) is changed to be coarser, and quantization is performed with the changed quantization step. By doing so, a bit range is kept within a predetermined width, to adjust a bit length per pixel to a fixed length in compression.
PTL 3 also intends to increase the number of continuous shots of the same image size with the same memory capacity. This is implemented by a method of determining a quantization width from a difference from an adjacent pixel and subtracting, from a pixel value to be compressed, an offset value uniquely derived from the quantization width, thereby determining a quantized value. PTL 3 thus provides a digital signal compression coding and decoding apparatus that realizes compression while ensuring a low coding processing load, without requiring more memory.